A recent study has published results on this topic, the bloating in the belly is not due to overeating, but to gluten intolerance.
The gluten is found in many grains like barley, rye and wheat too.
Oats do not have it in general, but have to be avoided. When they are processed and packed they might contact some grains.
This is Celiac disease when we come to contact with the gluten, the body responds in a strange way, the small intestines get irritated, we get pains, diarrhea, fatigue and bloating.
How to help a swollen stomach?
First, remove all gluten of your regime. Try this for a week. See if this is the problem.
If it improves, then you have to treat this gluten issue. If you still feel awful and bloated, see a doctor.
Try food tests for various intolerances, except the gluten maybe, and see what else you have to avoid.
Allergies to gluten are also common but in obese people. So a gluten free diet never hurt no one, remember.
Source and image source: healthyfoodandsporttips.com